This invention pertains to apparatus for transferring magnetically attractable toner from a magnetic image-storage medium to a toner-receiving medium. More specifically, it pertains to such an apparatus structured to apply, in combination, pressure and a magnetic field during toner transfer.
As indicated, this invention is primarily intended for use in a toner transfer system of the type having a conventional magnetic image-storage medium having a magnetizable facial expanse which is typically disposed on a rotatable drum surface. Facial expanse, containing magnetic images produced by a writing head, is transported past a counter-rotating toner-applicator cylinder having a layer of toner disposed thereon. The magnetic images attract the toner creating, thereby, toner images which it is desired to dispose on a separate toner-adherable receiving medium, such as paper. The paper is transported along a path adjacent the facial expanse for transferring the toner to the paper. Finally, the toner is fused to the paper.
Several different methods have been used to transfer toner from a drum to paper. A common method is to use a high-pressure platen to press the paper against the toner-holding facial expanse. Such a method is also sometimes used in combination with heat in order to increase the fusing of the toner onto the paper. Such processes have inherent disadvantages in that, due to the bending and distortion of the paper, the resulting image is distorted. It is also difficult to realign paper after it enters such a high-pressure nip region. Additionally, undesired toner residue often remains on the facial expanse.
Another method sometimes used is to transfer toner by magnetic tractive force as the paper is transported close to, yet spaced apart from, the drum facial expanse. The magnetic force is provided by disposing a magnetic pole of one polarity inside the drum and one of an opposite polarity on the opposite side of the paper from the drum. The outside pole adjacent the paper is placed nearer to the facial expanse than is the pole contained within the drum. The toner, being attracted to the outside pole, transfers to the paper.
Alternatively, a pair of opposite poles of a magnet have been known to be used adjacent the side of the paper opposite from the drum. The faces of such poles are disposed approximately normally to each other to create a generally rounded field in the area of desired toner transfer.
Such purely magnetic field transfer methods tend to cause blurred images and stray toner deposits due to inconsistent paths traveled in the space between the surfaces by the toner.
It is, therefore, a general object of the present invention to provide a toner transfer apparatus which overcomes the above-mentioned problems of the prior art.
More specifically, it is an object to provide a toner transfer apparatus which uses a combination of pressure and a well-defined magnetic field produced in an adjacent region of noncontact between the paper and the facial expanse.
It is further desired to provide an apparatus which provides a convenient method of D.C. erasing the facial expanse. This includes an apparatus which will condition the magnetic domains within the facial expanse prior to its being encoded with new images. Alternatively, where it is desired to repeat a given image, it is an object to provide an apparatus which transfers toner without altering the magnetic domains of the facial expanse.
An apparatus constructed as contemplated by the present invention includes a platen which presses the paper against the facial expanse of the drum as the two travel in adjacent commonly directed paths. This produces a region in which the paper and facial expanse are in contact and an adjacent region in which they are not in contact. A line of contact defines the boundary between the two regions.
Also included is at lease one magnet for producing a magnetic field which extends through the paper and facial expanse. Its stronger portion exists in the region of noncontact. Such a field is selectively energized to have two different operating states. In one state the magnetic force is less than the coercivity of the facial expanse. In the other, it exceeds that coercivity. In this latter state, the images stored in the facial expanse of the magnetic image-storage medium are erased prior to encoading the expanse with new images.
Thus, by sequentially applying a moderate pressure and a magnetic field, clear toner images are transfered.
These and additional objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiments.